The present invention generally relates to thermal imaging devices and systems, and, more specifically, relates to infrared detectors used in such thermal imaging systems.
Thermal imaging systems are used to convert a focused radiation image, principally in the infrared spectral region, of the temperature and thermal emissivity differences within a given scene into a visible picture. In such systems, the image may be scanned region-by-region over one or more detector devices or elements which transform the infrared radiation into an electrical signal. After suitable amplification and electronic processing, this signal can be used to energize an electrooptic transducer or display, such as a cathode ray tube, to provide a visual picture. The detector elements can be made from a semiconductor material, such as mercury cadmium telluride, so that the electrical signal is obtained as a change in resistivity due to free electrons and holes liberated from the bound molecular structure of the material by the infrared photons.
One such system employs a single detector element over which the whole image is scanned; but improved performance is obtained by using a plurality of detector elements, usually in a line (linear array). The image may be scanned and the elements arranged in such a way that each element samples a separate part of the same image, providing an overall improvement in signal-to-noise ratio as compared with a single element detector. This mode of operation is known as the "parallel scan" mode. Alternatively, the image may be scanned and the elements arranged in such a way that each region or spot of the image is focused onto each element in turn. The signals detected by the individual elements are added together so as to correlate with one another, but the noise associated with each is uncorrelated. Thus, this mode of operation, which is known as the "serial scan" mode, also provides an overall improvement in signal-to-noise ratio.
For both the parallel scan and serial scan systems, it is necessary to provide at least one electrical lead for each detector element, plus one common lead from the cooling vessel, and one preamplifier for each detector element. The number of electrical leads involved consequently makes encapsulation of the detector elements difficult and expensive to provide, and the number of preamplifiers affect the size, weight and cost of the system. A detector device which minimizes the number of electrical leads and preamplifiers required is shown in U.S. Pat. No. 3,995,159, issued Nov. 30, 1976, entitled "Thermal Imaging Systems", the inventor of which is Charles Thomas Elliott. Such patent describes a single three-electrode linear detector which replaces the conventional linear detector array in a serially scanned thermal imaging system. Such detector described in such patent comprises an elongated semiconductor/photoconductor strip of, e.g., the infrared-sensitive semiconductor material, mercury cadmium telluride. A bias current in the strip is arranged to give a photocarrier drift velocity in the strip which matches the image scanning velocity, thereby giving enhanced signal to noise ratio. The photocarrier pattern which constitutes the detected image is measured as a resistivity change between two readout electrodes positioned at one end of the elongated semiconductor/photoconductor strip. One problem associated with such elongated semiconductor/photoconductor strip is the limited spatial frequency response it provides.
It is, accordingly, a primary object of the present invention to provide a single detector in the form of an elongated semiconductor/photoconductor strip which provides improved frequency response bandwidth.